Food allergy is an exaggerated immune response to dietary components, usually proteins. Manifestations vary widely and can include atopic dermatitis, GI or respiratory symptoms, and anaphylaxis. Diagnosis is by history and sometimes allergen-specific serum IgE testing, skin testing, and/or elimination diets. Treatment is with elimination of the food that triggers the reaction and sometimes oral cromolyn.

Etiology

Almost any food or food additive can cause an allergic reaction, but the most common triggers include

In infants and young children: Milk, soy, eggs, peanuts, and wheat

In older children and adults: Nuts and seafood

Cross-reactivity between food and nonfood allergens exists, and sensitization may occur nonenterally. For example, patients with oral allergies (typically, pruritus, erythema, and edema of the mouth when fruits and vegetables are eaten) may have been sensitized by exposure to pollens that are antigenically similar to food antigens; children with peanut allergy may have been sensitized by topical creams containing peanut oil used to treat rashes. Many patients who are allergic to latex are also allergic to bananas, kiwis, avocados, or a combination.

In general, food allergy is mediated by IgE, T cells, or both. IgE-mediated allergy (eg, urticaria, asthma, anaphylaxis) is acute in onset, usually develops during infancy, and occurs most often in people with a strong family history of atopy. T-cell–mediated allergy (eg, dietary protein gastroenteropathies, celiac disease) manifests gradually and is chronic; it is most common among infants and children. Allergies mediated by both IgE and T cells (eg, atopic dermatitis, eosinophilic gastroenteropathy) tend to be delayed in onset or chronic.

Eosinophilic gastroenteropathy

This unusual disorder causes pain, cramps, and diarrhea with blood eosinophilia, eosinophilic infiltrates in the gut, and protein-losing enteropathy; patients have a history of atopic disorders.

Symptoms and Signs

Symptoms and signs of food allergies vary by allergen, mechanism, and patient age. The most common manifestation in infants is atopic dermatitis alone or with GI symptoms (eg, nausea, vomiting, diarrhea). Children usually outgrow these manifestations and react increasingly to inhaled allergens, with symptoms of asthma and rhinitis; this progression is called atopic march. By age 10 yr, patients rarely have respiratory symptoms after the allergenic food is eaten, even though skin tests remain positive. If atopic dermatitis persists or appears in older children or adults, its activity seems largely independent of IgE-mediated allergy, even though atopic patients with extensive dermatitis have much higher serum IgE levels than atopic patients who are free of dermatitis.

When food allergy persists in older children and adults, the reactions tend to be more severe (eg, explosive urticaria, angioedema, even anaphylaxis). In a few patients, food (especially wheat and shrimp) triggers anaphylaxis only if they exercise soon afterward; mechanism is unknown. Food may also trigger nonspecific symptoms (eg, light-headedness, syncope). Occasionally, cheilitis, aphthous ulcers, pylorospasm, spastic constipation, pruritus ani, and perianal eczema are attributed to food allergy.

Diagnosis

Severe food allergy is usually obvious in adults. When it is not or when it occurs in children (the most commonly affected age group), diagnosis may be difficult, and the disorder must be differentiated from functional GI problems. For diagnosis of celiac disease, see Celiac Disease : Diagnosis.

In either case, a positive test does not confirm a clinically relevant allergy. Both tests can have false-positive or false-negative results. Skin testing is generally more sensitive than the allergen-specific serum IgE test but is more likely to have to false-positive results. The skin test provides a result within 15 to 20 min, much more quickly than the allergen-specific serum IgE test. If either test is positive, the tested food is eliminated from the diet; if eliminating the food relieves symptoms, the patient is reexposed to the food (preferably in a double-blind test) to see whether symptoms recur. (See also the National Institute of Allergy and Infectious Diseases (NIAID) medical position statement:
Guidelines for the diagnosis and management of food allergy in the United States
.)

For the latter diet, no foods or fluids may be consumed other than those specified. Pure products must always be used. Many commercially prepared products and meals contain an undesired food in large amounts (eg, commercial rye bread contains wheat flour) or in traces as flavoring or thickeners, and determining whether an undesired food is present may be difficult.

If no improvement occurs after 1 wk, another diet should be tried; however, T-cell–mediated reactions may take weeks to resolve. If symptoms are relieved, one new food is added and eaten in large amounts for >24 h or until symptoms recur. Alternatively, small amounts of the food to be tested are eaten in the clinician’s presence, and the patient’s reactions observed. Aggravation or recrudescence of symptoms after addition of a new food is the best evidence of allergy.

Allowable Foods in Elimination Diets*

Food

Diet No. 1

(No beef, pork, fowl, milk, rye, or corn)

Diet No. 2

(No beef, lamb, milk, or rice)

Diet No. 3

(No lamb, fowl, rye, rice, corn, or milk)

Cereal

Rice products

Corn products

None

Vegetables

Artichokes, beets, carrots, lettuce, spinach

Asparagus, corn, peas, squash, string beans, tomatoes

Beets, lima beans, potatoes (white and sweet), string beans, tomatoes

Meats

Lamb

Bacon, chicken

Bacon, beef

Flour (bread or biscuits)

Rice

Corn, 100% rye (ordinary rye bread contains wheat)

Lima bean, potato, soybean

Fruits

Grapefruit, lemons, pears

Apricots, peaches, pineapple, prunes

Apricots, grapefruit, lemons, peaches

Fat

Cottonseed oil, olive oil

Corn oil, cottonseed oil

Cottonseed oil, olive oil

Beverages

Coffee (black), lemonade, tea

Coffee (black), lemonade, tea

Coffee (black), lemonade, juice from approved fruit, tea

Miscellaneous

Cane sugar, gelatin, maple sugar, olives, salt, tapioca pudding

Cane sugar, corn syrup, gelatin, salt

Cane sugar, gelatin, maple sugar, olives, salt, tapioca pudding

*Diet No. 4: If symptoms persist when patients are following any of the above 3 elimination diets and diet is still suspected, daily diet may be restricted to an elemental diet (using extensively hydrolyzed or amino acid–based formulas).

Treatment

Food elimination diet

Sometimes oral cromolyn

Sometimes corticosteroids for eosinophilic enteropathy

Treatment of food allergies consists of eliminating the food that triggers the allergic reaction. Thus, diagnosis and treatment overlap. When assessing an elimination diet’s effect, clinicians must consider that food sensitivities may disappear spontaneously.

Oral desensitization (by first eliminating the allergenic food for a time, then giving small amounts and increasing them daily) and immunotherapy using sublingual drops of food extracts are under study.

Oral cromolyn has been used to decrease the allergic reaction with apparent success. Antihistamines are of little value except in acute general reactions with urticaria and angioedema. Prolonged corticosteroid treatment is helpful for symptomatic eosinophilic enteropathy.

Patients with severe food allergies should be advised to carry antihistamines to take immediately if a reaction starts and a prefilled, self-injecting syringe of epinephrine to use when needed for severe reactions.

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